1. Classification of PSA oxygen generation molecular sieve
According to the separation mechanism of gas components by PSA method, adsorbents can be divided into kinetic selective adsorbents and equilibrium selective adsorbents. The former (such as carbon molecular sieve) is separated based on the difference in the diffusion rate of adsorbate molecules in the pores of the adsorbent, while the latter (such as zeolite molecular sieve) is separated by the force of adsorbate molecules in the pores of the adsorbent.
The adsorbents used in the early industrial PSA for oxygen generation were mainly carbon molecular sieves and zeolite molecular sieves. Through the research on the PSA oxygen generation process, it is found that the effect of using carbon molecular sieve to produce oxygen is not good, and the purity of the obtained oxygen enrichment is low. Therefore, carbon molecular sieve is rarely used in the industry to produce oxygen. At present, the most widely used oxygen generation adsorbent on the market is zeolite molecular sieve, which is a balanced selective adsorbent, including X-type zeolite (LiX, NaX, CaX) and A-type zeolite (CaA).
2. Zeolite molecular sieve generals
Zeolite molecular sieve is essentially an aluminosilicate compound with a cubic lattice, which is mainly composed of silicon and aluminum connected by oxygen bridges to form a framework structure. The basic structural unit is formed by 4 oxygen anions surrounding a smaller silicon or aluminum ion. Tetrahedron. The role of sodium ions or other cations is to supplement the deficiency of the positive charge of the aluminum oxide tetrahedron. Each oxygen anion is shared by another aluminum-oxygen or silicon-oxygen tetrahedron to extend the lattice three-dimensionally. The exposed cations in the crystal lattice make the zeolite molecular sieve have stronger adsorption capacity. These cations act as local strong positive charge points and electrostatically attract the cathodes of polar molecules. Changing the cation species in the zeolite molecular sieve is one of the important methods to change the adsorption performance of the adsorbent.
The change of cations in zeolite molecular sieves is accomplished by ion exchange reaction. The reaction is generally carried out in an aqueous solution. Non-aqueous ion exchange methods (generally using organic solvents) or molten salt exchange methods can be used to change the cations in zeolite molecular sieves. sex. At a certain temperature, the zeolite molecular sieve (usually Na zeolite) to be exchanged is contacted with the metal salt solution to be exchanged, and the cations in the molecular sieve are exchanged with the cations in the metal salt solution. In order to obtain a certain degree of exchange, continuous exchange can be carried out in a special exchanger (usually a fixed bed) or multiple exchanges can be carried out in a general container. In order to complete the exchange, it is often necessary to perform an intermediate roasting process to redistribute the cations in the crystal lattice to positions that are easy to exchange, so as to facilitate the next exchange. In general, cation exchange will not have a great impact on the framework structure of zeolite molecular sieve, but it can significantly change the adsorption performance of zeolite molecular sieve. At present, the modification of PSA oxygen generation adsorbents is also mainly focused on ion exchange modification.
3. Zeolite molecular sieve development
The adsorption capacity of zeolite molecular sieve for nitrogen and the separation coefficient of nitrogen and oxygen are the most critical factors that determine the scale and technical and economic indicators of the PSA oxygen generation device. Therefore, research on adsorbents for oxygen generation should focus on improving the selective adsorption performance of adsorbents for nitrogen and oxygen. The PSA oxygen generation has been changed from CaA zeolite molecular sieve to CaX and NaX molecular sieves, and later the use of high lithium low silicon aluminum X-type LiLSX molecular sieve with high oxygen recovery rate and greatly reduced energy consumption.
5A (CaA) molecular sieves and 13X (NaX) molecular sieves are general molecular sieves that can be used for PSA oxygen generation. As of the end of the 1990s, oxygen production devices using CaA and NaX molecular sieves occupied a dominant position in the market. However, their nitrogen adsorption capacity is small. When pressure swing adsorption is performed, the oxygen adsorption capacity is relatively high, but the nitrogen-oxygen separation coefficient is small, and the oxygen recovery rate is low. Although the nitrogen adsorption capacity of the CaX type molecular sieve obtained after Ca2+ replacement has increased, the nitrogen-oxygen separation coefficient is still low, generally around 3.
The NaX type molecular sieve modified by Li+ ions (ie, LiX molecular sieve) has large air handling capacity and high oxygen yield. At the same oxygen production scale, the amount of LiX molecular sieve is small. The amount of air required is small. The size of the static equipment required is small, and the load of the moving equipment is small. This is because Li+ has the smallest radius among all metal ions, that has a higher charge density and is very high polarizability, so the force between Li + and N2 is greater. The 13X molecular sieve modified by Li + has stronger adsorption performance and is currently the best performance among pressure swing adsorption oxygen generation adsorbents9. Another advantage of LiX molecular sieve is that the optimal operating pressure ratio is lower (that is, the ratio of adsorption pressure to desorption pressure), and the operating pressure ratio is an important factor that determines the energy consumption of PSA oxygen production.
In recent years, due to many problems, 5A molecular sieves and 13X molecular sieves have become obsolete, and with the in-depth research on the modification of zeolite molecular sieves, LiX molecular sieves have gradually taken the leading position in the market.
In 1989, the 700 m3/h oxygen generation unit of the US Prexair Company using LiX molecular sieve was put into operation in Canada, marking the pressure swing adsorption oxygen production technology has entered a new development period. Subsequently, countries around the world set off an upsurge in the study of LiX molecular sieves.